The invention concerns an X-ray optical system with two X-ray mirrors for imaging an X-ray source on a target region.
A system and method of this type is known from Paul Kirkpatrick and A. V. Baez, J. Opt. Soc. Am. 38, 9 (1948).
The above-mentioned article describes in detail the principal function of an arrangement of this type. It comprises two concave X-ray mirrors which are disposed one behind the other such that the plane of reflection of the first mirror is perpendicular to the plane of reflection of the second mirror. The X-ray radiation which is incident on the first mirror at a very flat angle, is focused in a first coordinate direction and is incident on the second mirror at a likewise flat angle where it is focused in a second coordinate direction perpendicular to the first coordinate direction. In this manner, one obtains X-ray radiation which is focused in two coordinate directions with the ray divergences being at least partially corrected.
The two concave X-ray mirrors may have cylindrical, elliptical or parabolic, curved surfaces. In particular, the use of parabolic mirrors also permits rendering the incident X-ray radiation parallel.
A disadvantage of this conventional Kirkpatrick-Baez arrangement is the considerably limited region of acceptance of the two mirrors. Due to the fact that the Bragg condition must be met for both mirrors, only a surface is imaged which is considerably smaller than the visible radiating overall surface of the X-ray source (approximately 1/100).
U.S. Pat. No. 6,041,099 proposes an improvement to the Kirkpatrick-Baez arrangement, i.e. a one-piece mirror with two reflecting surfaces disposed at 90° with respect to each other (referred to as a “side-by-side” arrangement). This arrangement is intended to approximately double the reflected intensity of the incident X-ray radiation. The configuration is more compact than the classical Kirkpatrick-Baez arrangement having two mirrors disposed in series.
The use of multi-layer mirrors in connection with a Kirkpatrick-Baez arrangement is described in an article by J. Underwood in Applied Optics, Vol. 25, No. 11 (1986).
To give an impression of the magnitudes of the quantities of interest, it should be noted that the angle of acceptance of typical multi-layer mirrors is in the region of 1 mrad and typical foci in the region of a few centimeters. The electron focus of the X-ray source varies in a linear region between 10 μm and a few millimeters. The angle of acceptance of one mirror has a minimum linear dimension in the region of a few 10 μm and is typically striped. On the other hand, conventional X-ray samples have linear extensions in the region of 100 μm to a few millimeters, typically several tenths of a millimeter.
A problem of X-ray optical systems of this type is the relatively low intensity reflected by the mirror arrangement due to the Bragg condition of the focused X-ray radiation, compared to the theoretically possible yield given by the size of the radiating surface of the X-ray source. Moreover, due to the surface size of the sample to be examined, an increased X-ray radiation yield is desirable.
In view of the above, it is the underlying purpose of the present invention to present an X-ray optical system with the above-mentioned features which increases the intensity of the focused X-ray radiation on the sample for a given X-ray source emission power and with as few, technically simple modifications as possible.